The present invention relates to a system for the emergency cooling of the core of a pressurized water nuclear reactor.
As is known, in this reactor type, the core is submerged in ordinary hot water at approximately 300.degree. C., under a high pressure of approximately 155 bars, it being of vital importance that in all circumstances the core remains submerged in this water during operation or after reactor shutdown. Furthermore, it is important that the water can always be cooled to prevent any abnormal heating of the fuel which, in extreme cases, with all or part of the core dried up, could be subject to a temperature rise and at least partly melt causing unacceptable, dangerous and irreversible damage.
In the presently known installations, there is generally an emergency injection system for preventing the draining of the vessel containing the core or connecting the core to the primary circuit in the case of a leak. There is also an emergency and shutdown cooling system enabling the removal of the residual power and the cooling of the primary circuit, so that cold shutdown can take place.
The emergency injection system generally comprises high pressure pumps, medium pressure accumulators and low pressure pumps.
Up to medium pressure, the emergency and shutdown cooling system uses a system for the water supply and steam extraction from generators, as well as a direct cooling system for the primary circuit using a special exchanger for the final cooling.
The pressurized water-filled accumulators are linked with the primary cooling circuit of the reactor via isolating valves sensitive to a given lower value of the pressure in the primary circuit. The water filling the accumulators is pressurized by nitrogen.
The known device has a number of disadvantages. Thus, the injection of water from the emergency cooling accumulators of the core does not necessarily take place at the most favourable moment. Thus, when the pressure drop in the primary circuit results from a relatively unimportant piping break or failure which tends to spontaneously close again, there can be a significant drainage or emptying of the primary circuit without an adequate drop in the primary pressure to start the emptying of the accumulators or the emergency low pressure injection. The primary circuit can then be filled with emergency high pressure pumps provided for this purpose, but they draw their supplies from tanks with a limited capacity and in no case constitute the final cooling sources. In addition, the emptying of the accumulators is linked with a primary circuit pressure value and not with a critical value of the water level in the vessel, which is the most important physical quantity with regards to the need for filling the primary circuit with water.
Moreover, the injection of water into the primary circuit is generally terminated by an injection into it of nitrogen used for pressurizing the water in the accumulators. This gas can then be trapped either in the inverted U-tubes of the steam generators, or in the upper parts of the hot branches of the primary circuit when the steam generators are of the "single pass" type, in which the primary fluid enters the upper end of the generator and leaves at its lower end. Thus, in both cases there is a reduction of the efficiency of the cold source formed by the steam generators.